In the process of rotary drilling a well, a drilling fluid or mud is being circulated down the rotating drill pipe, through the bit, and up the annular space between the pipe and the formation or steel casing, to the surface. The drilling fluid performs different functions such as removal of drilled cuttings from the bottom of the hole to the surface, suspension of cuttings and weighting material when circulation is interrupted, control of subsurface pressures, maintaining the integrity of the wellbore until the well section is cased and cemented to suspend cuttings and weighting material when the circulation is interrupted, control subsurface pressure, isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, cool and lubricate the drill string and bit, maximise penetration rate etc.
The required functions can be achieved by a wide range of fluids composed of various combination of solids, liquids and gases and classified according to the constitution of the continuous phase mainly in two groupings : aqueous (water-based) drilling fluids, and non-aqueous (mineral oil or synthetic-base) drilling fluids, commonly `oil-base fluids`.
For all fluid types, it is desirable to minimise the permeability damage in drilling reservoir formations when in fact, during the operations for drilling and completing hydrocarbon-bearing formations (reservoirs), an overbalance pressure is often applied which causes fluid loss from the wellbore into the reservoir rock. This filtration process causes build-up of a low permeability filtercake, comprised of the solid phases present in the fluid. The presence of this cake can be detrimental to the production of hydrocarbons from the reservoirs.
It is well known to use wash fluids to break-up and dissolve (wholly or partly) the filtercake. The fluids used to drill and complete the reservoir are generally polymer solutions with various additives including calcium carbonate, clay and salt. The polymer solutions are typically comprised of one or more of the following: bioploymers (Xanthan, Scleroglucan), starches and celluloses (hydroxyethyl cellulose[HEC], polyanionic cellulose [PAC]). The filtercakes are therefore comprised of these components, plus drilled solids picked up during the drilling phase.
It is preferable to remove as many of these components as possible from the filtercake which has built up on the producing formation. There are various chemicals and methods used for removal of these cakes. These include: aqueous solution with oxidiser present, acid wash solution (hydrochloric acids, organic acids such as formic and acetic), combinations of acid and oxidiser, and aqueous solution with enzymes. The oxidisers and enzymes attack the polymers whilst the acids mainly attack the carbonate and polymers. For the majority of the recent drilling fluids used in reservoir sections, the acids do not substantially break-down the polymeric component and in fact are used for removal of the calcium carbonate. This facilitates the back-production of the filtercake through the screens and limits damage of the completion.